Note the 2 answers you received are different. both "you bet" and "I bet" are slang, so they have different meanings depending on the situation. The most commont: If you describe your feelings with some kind of judgement, like, "that was really fun!", and they say "I bet!", that means they are agreeing with your judgement. In this example, the "I bet" answer is similar to saying "yes, it sounds like it was fun". Another example: If you say "it was really hard to finish that homework" and they say, "I bet", they mean "yes, it sounds like it was hard for you to finish your homework". If you say "I am really mad that I lost that game" and they say "I bet", that means, "yes, I believe you are really mad that you lost that game." Thus, it is agreement with both positive and negative statements. "You bet" usually means "you're welcome". There are exceptions to this, but this is the most common meaning, especially if you say "thank you" and they answer with "you bet"

I bet is a short form of I'll bet!" means hearty agreement or to indicate that you are sure something is true. You bet! is a casual way of saying "You're welcome. You bet!" is similar to saying, "No problem."

If you're not talking about gambling... "You bet!" = Of course! "I bet." = I don't believe you. (Often accompanied with rolling eyes emphasizing your disbelief about what was said.)